A novel technique for projection-type electron-beam lithography

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					                                                                                                            10.1117/2.1200812.1396




A novel technique for
projection-type electron-beam
lithography
Ki-Bum Kim


Atomic images obtained from high-resolution transmission electron
microscopy can serve as a template for producing a variety of useful
nanostructures.

One of the challenges of nanotechnology is to fabricate
nanometer-scale features of uniform size and shape, and with
acceptable speed. Such materials will have a wide range of
applications, for example, in developing the future generation
of electronic, optical, magnetic, and biological devices, due to
their novel and significantly improved physical, chemical, and
biological properties. Most top-down approaches to making
nanoscale features attempted to date, such as electron-beam
lithography and probe tip-based lithography, show good con-
trollability but unsatisfactory throughput. In contrast, bottom-
up methods, such as gas-phase and liquid-phase synthesis of
quantum dots and quantum wires, pose challenges in control-
ling size and shape. Here, we propose a novel technique for pro-
ducing various nanostructures based on crystalline lattice im-
ages obtained from high-resolution transmission electron mi-           Figure 1. HRTEM image of a silicon (Si) [110] crystallographic zone
croscopy (HRTEM).                                                      axis.
   Since the invention of TEM by Max Knoll and Ernst Ruska in
1931,1 the resolution of the microscope has undergone drastic
                                                                       whether these images can be used as a template to actually make
improvement, currently extending down to tenths of nanome-
                                                                       nanoscale dot features. This is precisely the problem we set out
ters. In the 1950s, James Menter2 managed to obtain lattice-
                                                                       to solve.
fringe images using phase-contrast interference signals between
                                                                          In HRTEM, the image signal is first formed at the image plane
the transmitted beam and diffracted beam. As a result of these
                                                                       of the objective lens and then magnified further using a series of
advances, imaging the crystalline lattice of many different ma-
                                                                       intermediate and projection lenses to reveal the small-size fea-
terials to investigate their atomic structures has become com-
                                                                       tures. Typically, observing lattice-fringe images requires magni-
mon practice. For instance, Figure 1 shows a high-resolution
                                                                       fying up to several hundred thousand times. In our case, how-
phase-contrast image of silicon observed at the [110] crystallo-
                                                                       ever, we are interested using the image to make nanoscale pat-
graphic zone axis. The dots represent double columns of silicon
atoms separated by 0.33nm. Such atomic-scale images, achieved
using HRTEM, may well constitute the ultimate in what hu-
mans can currently observe in terms of size. The next question is                                                 Continued on next page
                                                                                                     10.1117/2.1200812.1396 Page 2/3




Figure 2. Atomic image projection electron-beam lithography hard-
ware. (a) Design diagram. (b) Ray diagram.


terns. Consequently, the magnification needed is somewhere be-
tween several ten to several hundred times, obtainable in one of
the conjugated image planes situated below the objective lens.
   To verify our concept experimentally, we modified a field-
emission TEM with a 200kV accelerating voltage.3 Figure 2              Figure 3. (a) Dot and line array patterns from a silicon mask and (b)
shows the hardware design diagram with the corresponding ray           various patterns obtained from a β-Si3 N4 mask.
diagram. The sample stage is installed at the image plane of the
objective lens, called a lithography plane, and the patterning lens
                                                                       enon, we show just a few of the patterns obtained from crys-
is inserted between the objective lens and sample stage to control
                                                                       talline β-Si3 N4 used as a mask material.
the magnification of the image signal at the sample. The pattern-
                                                                          We have demonstrated that our method does not produce
ing magnification can be varied from 50 to 300 times by adjusting
                                                                       only nanoscale dots and lines. Much more complicated struc-
the current of the patterning lens.
                                                                       tures can be fabricated using the crystalline lattice images ob-
   Figure 3 shows some of the nanoscale patterns fabricated on
                                                                       served with HRTEM. The question now is what kinds of prop-
a silicon substrate using single-crystalline silicon and polycrys-
                                                                       erties these nanostructure patterns will reveal. Our group is ac-
talline beta-silicon nitride (β-Si3 N4 ) as mask materials. The mask
                                                                       tively working on identifying novel functionalities from the pat-
sample was made using a conventional TEM sampling method
                                                                       terns.
of dimpling and ion-beam milling. Hydrogen silsesquioxane is
used as an electron-beam resist. After developing the resist, we
etched the silicon substrate using hydrogen chloride reactive-         Author Information
ion etching. Note that the high-resolution lattice image we ob-
served in TEM is a 2D projection image of atom arrangements            Ki-Bum Kim
in three dimensions. Accordingly, one can obtain various pat-          Materials Science and Engineering
terns simply by changing the observation zone axis of a given          Seoul National University
material. For instance, Figure 3(a) shows the nanoscale dot (ob-       Seoul, Korea
served at exactly the [110] zone axis) and line features (observed
by slightly tilting the sample from the [110] zone axis) obtained      Ki-Bum Kim has been a professor of materials science and engi-
in a silicon sample. Note, too, that there are an enormous vari-       neering at Seoul National University since 1992. He has an MS
ety of crystalline materials in nature that produce many differ-       degree in metallurgical engineering from Seoul National Univer-
ent kinds of high-resolution images. To illustrate this phenom-

                                                                                                                   Continued on next page
                                                                                             10.1117/2.1200812.1396 Page 3/3



sity and a PhD degree in materials science and engineering from
Stanford University. He has been a research scientist at Philips
Research Laboratory and Applied Materials Inc. He has coau-
thored over 100 journal publications.

References
1. M. Knoll and E. Ruska, Das Elektronenmikroskop, Z. Physik 78, p. 318, 1932.
2. J. W. Menter, The direct study by electron microscopy of crystal lattices and their im-
perfections, Proc. Roy. Soc. London A 236, p. 119, 1956. doi:10.1098/rsta.1973.0104
3. H. S. Lee, B. S. Kim, H. M. Kim, J. S. Wi, S. W. Nam, K. B. Jin, Y. Arai, and K.
B. Kim, Electron beam projection nanopatterning using crystal lattice images obtained
from high-resolution transmission electron microscopy, Adv. Mater. 19, p. 4189, 2007.
doi:10.1002/adma.200701119




                                      c 2008 SPIE